Morphological Evaluation of Maxillary Premolar Canals in Iranian Population: A Cone-Beam Computed Tomography Study

Statement of the Problem: A thorough knowledge of the morphological and anatomical variations of root canal system can help identify all root canals, adequate instrumentation, and consequently leads to a successful endodontic treatment. The knowledge of root morphology can influence the outcome of root canal therapy. Purpose: The aim of this study was to investigate the morphology of maxillary premolar canals in Iranian population by analyzing cone-beam computed tomography (CBCT) scans. Materials and Method: This cross-sectional retrospective study consisted of CBCT of 280 patients over 16 years of age. The position, number of roots, and root morphology of maxillary premolars were inspected. The root canal configurations of maxillary premolar teeth were also analyzed according to the Vertucci classification. The statistical analyses wee performed using chi-square test. Significance level was set at p < 0.05. Results: In the present study, of 462 maxillary first premolars, 8 (1.73%) teeth had three roots, 222 (48.05%) teeth had two roots, and 232 (50.22%) were single-rooted. In the second maxillary premolar group, of 400 premolars, 2 (0.5%) teeth had three roots, 34 (8.5%) teeth were two-rooted, and 364 (91%) were single-rooted. The most root canal configurations were type IV (71.64%) and type I (63%) in maxillary first and second premolars, respectively. Among females, single rooted premolars were the most prevalent (56.83%), and among males, two-rooted premolars were the most prevalent (57.61%). Conclusion: This study provided information about the root canals of maxillary premolar teeth for Iranian subpopulation leading to more optimal diagnosis and treatment planning for the endodontists. According to the findings, the complexity of root canal system and the number of roots were less common in females compared to males.


Introduction
Success in root canal treatment depends entirely on cleaning and disinfection of all root canals in a teeth [1].
Inadequate knowledge of the root canal anatomy results in missing and insufficient cleaning of root canals. The complexity of root canal anatomy is the cause of most root canal treatment failures including missing root canals [2]. Among different groups of teeth, root canal treatment of premolar teeth has always been a challenge for clinicians [3]. Many studies of different populations have reported an extremely diverse and complex root and canal morphology for the premolar teeth showing additional canals and roots [4][5][6]. Several studies have reported that maxillary premolars have a highly variable internal canal configuration, which can vary according to race and geographic origin [4][5][6][7].
Significant anatomical variation has also been observed in the maxillary second premolar. Vertucci, in a research performed on maxillary second premolars, reported a prevalence of 1% for three-rooted second premolars [13]. The incidence of three canals in maxillary premolars has also been reported to vary from 0% to 10% [14][15].
One of the factors that can affect the results of prevalence studies on the morphology of teeth in diverse populations is the various in-vivo and in-vitro methods used to identify root canal morphology in different studies [16][17].
Recently, the application of in-vivo methods such as cone-beam computed tomography (CBCT), that can create a three-dimensional image in the axial, sagittal and coronal dimensions with the precision of canal staining and clearing techniques and without the limitations of conventional radiography such as distortion and superimposition of images, has become increasingly popular [18][19][20][21].
In addition, when using CBCT, patients' demographic data such as age and gender as well as the symmetry of specific morphologies can be verified while this may not be feasible in the in-vitro methods performed on extracted teeth [22].
Besides the investigation methods, other factors such as sex [7,[23][24], age [25][26], race, or ethnicity [26], and the geographical location [6] can affect the root canal system especially in the premolar teeth and subsequently can influence the morphological complexity of the canal. Therefore, clinicians should be aware of the anatomy of this group of teeth to be able to use appropriate therapeutic techniques to achieve higher success rate in root canal treatment.
Considering the vastness of different races of people in Iran, there is a need to study of number and root morphology of maxillary premolars in the south of Iran.
Therefore, the aim of this study was to investigate the root and canal morphology of maxillary premolar teeth in relation to sex and position in an Iranian subpopulation in the south of the country by using CBCT from 2015 to 2018. The CBCTs enrolled in this study have been collected from the patients between 16 and 60 years of age who had intact maxillary premolars with completed root formation and without root canal treatment. Teeth with any signs of filling, external, and internal root canal resorption, periapical lesions, post, crown or calcification were excluded from the study. In addition, the images of teeth with compromised anatomy and poor quality images were withdrawn from the study.

Materials and Method
All images were prepared using Promax 3Dmax Images were evaluated using Romexis imaging software version 3.8.2 on a 23-inch monitor in a dim light. Using axial, coronal, and sagittal planes, all of the CBCT images were inspected retrospectively by a calibrated endodontist and senior dental student who were trained to interpret images independently, with a twoweek interval between the assessments. Prior to the experiment, the investigators graded 50 samples of CBCT images. If any disparity was seen between the opinions, a radiologist to reach a consensus evaluated the images.
To assess the intraexaminer reliability, a re-assessment was performed one month following the first session.
The data obtained from CBCTs were recorded as follows: 1. The number of roots from the axial plane based on the classification of Pecora et al. [31] 2. The number of root canals in each root 3. The root canal configuration (based on the Vertucci classification) (Figure 1) [32] 4. Position of the teeth in jaws for bilateral symmetry evaluation 5. Patients' gender based on demographic records of CBCTs

Data Analyses
Data were analyzed using SPSS software version 18 (SPSS Inc, Chicago, IL, USA). Chi-square test was used to study the frequency of various morphologies between different sex and root morphology groups. Quantitative data was described with mean plus standard deviation and qualitative data was presented using frequency (percent). Significance level was set at p< 0.05. Differences in tooth position (right or left) were determined by using the X 2 test and were considered significant if the p Value was <.05. Intraexaminer and interexaminer agreements were calculated by using the Cohen kappa coefficient.

Results
Of 280 CBCT images, 862 bilateral maxillary premolars were included in the study, of which 462 were maxillary first premolar and 400 were maxillary second premolar.
In this study, the number of roots recorded for the maxillary first and second premolars has been up to three roots, and the canal configuration of these teeth were classified as type I to type VIII (Table 1).

Number of roots
Of the 462 maxillary first premolars, 232 teeth (50.22%) were single-rooted presenting the highest prevalence in this subpopulation; 222 teeth (48.05%) had two roots, and 8 teeth (1.74%) had three roots. From the tworooted maxillary first premolars, in 45.95% (n=102) cases, the roots were separated into two independent roots from the floor of pulp chamber, and in 54.05% (n=120) cases, the roots were separated at different levels of the root.

Type of roots
Overall, the most common canal configuration for the maxillary first premolar was type IV (n = 331; 71.64%), followed by type II (n = 70; 15.15%) and type I (n = 41; 8.87%). Other types made up 4.33% of all canal configurations, which is a low percentage.
In the single-rooted maxillary first premolars, 115 teeth (49.57%) had one foramen in root and 117 (50.43%) had two foramens. While all the two-rooted maxillary first premolars had one foramen in each root, there was only one case with two foramens in the buccal root and one foramen in the palatal root. In all three-rooted first premolars, there was one foramen in each root.

Number of roots
Of the 400 maxillary second premolars, the highest prevalence was observed for single roots (n= 364; 91%), followed by two roots (n= 34; 8.5%) and three roots (n= 2; 0.5%). From the two-rooted maxillary second premolars, in 38.24% (n=13) cases, the roots were separated into two independent roots from the floor of pulp cham-ber, and in 61.76% (n=21) cases, the roots were fused.

Figures 2 to 5 show a selection of root numbers detected
in different sections.

Type of roots
As a general finding, the most common canal configuration for the maxillary second premolar was type I (n= 252; 63%), followed by type II (n= 78; 19.5%) and type IV (n = 57; 14.25%).

Number of apical foramens
The second maxillary premolar with 1, 2 or 3 apical  In Table 3 The overall Cohen kappa value for intraexaminer agreement was 0.998. In brief, there was a very good intraexaminer and interexaminer agreement.

Discussion
One of the most common causes of failure in root canal treatment, especially in premolars, is missed canals due to the inadequate knowledge of the specific morphologies in this group of teeth [2]. This study was designed to evaluate the prevalence of extra roots and canals in maxillary premolars among an Iranian subpopulation using CBCT. Of various methods of studying the canal morphology, CBCT has been selected in the present study. Neelakantan et al. [19] revealed that CBCT can show the root canal system with the precision of staining and clearing technique and more accurately than the conventional radiography. Moreover, other studies also found that CBCT was more accurate in detecting additional canals than intra-oral radiography [20][21]33].
In addition to the accuracy of this method, CBCT is a non-invasive technique, which, unlike in-vitro methods, does not require extracted teeth and thus, can provide better information concerning the patients' gender, age, and even teeth symmetry, which is not possible to obtain by other laboratory methods [19].     In studies on other Iranian subpopulations, like an invitro study by Ketabi et al. in Isfahan [28], single rooted maxillary first premolars were more prevalent.
In addition, Tofangchiha et al. [27] in Ghazvin showed that single rooted maxillary first premolars were more prevalent. Similar to our study, Tofangchiha et al.
also used CBCT in their research. However, in a study by Ghate et al. [29] two-rooted maxillary first premolars were more prevalent.
Similar to our study, in studies on other nationalities, Tian et al. [11] and Celikten et al. [34] showed that single rooted maxillary first premolars were more prevalent. However, in contrast to our findings, Abella et al. [7] and Alqedairi et al. [35] reported a more prevalent maxillary first premolar with two roots. In this study, the prevalence of two-rooted maxillary first premolars was (n=222; 48.05%). Only one (0.45%) of the two-rooted first premolars showed three canals, with two canals being in the buccal root and one canal located in the palatal root. This finding was not observed in recent morphological studies using CBCT as they reported a single canal in each root in the two-rooted and threerooted first premolars [7,11,[34][35][36]. Only Nazeer et al. [37] have observed three canals in the single-rooted and two-rooted premolars in its population.
Lipski et al. [38] first accredited the term "ridiculous" to the three-rooted first premolars, which is a characteristic of the Caucasian race. In the present study, the prevalence of maxillary first premolars with three distinct roots and three canals was observed to be 1.74%, which is similar to the findings of Ketabi et al. [28] who also reported a prevalence of 1.85% for the three-rooted  [7,11,[34][35][36][37]39]. Only Nazeer et al. [37] and Yang et al. [40] did not observe three-rooted maxillary first premolars in their studies. In general, single rooted maxillary second premolar is the most common type of this group of teeth reported in recent studies (Tables 4   and 5). Similarly, in the present research, the prevalence of second premolars with a single root was observed to be (n= 364; 91%). In our study, two cases of maxillary first premolars with two roots were observed to have three canals. This finding has not been previously reported in other studies. Moreover, in this study, the prevalence of three roots with three distinct canals was reported to be (n= 2, 0.5%), which was consistent with the range reported in previous studies (0-1%) [40].
Partovi et al. [41] reported no cases of three roots with three canals in their in vitro study. Nazeer et al. [37] also reported a prevalence of 0% for three-rooted maxillary second premolars in their study.  Furthermore, in the present study, type VIII was observed in both two-rooted and three-rooted premolars.

Awareness
In a study by Nazeer et al. [37], type VIII was reported in single-rooted and two-rooted premolars. These differences observed in the number of roots and canals may be due to the different method of analysis, the sample size, and the geographically diverse populations.
Felsyprenila et al. [45] has reported a bilateral symmetry in 81.5% of the maxillary first and second premolars. In this study, all the first and second premolars were selected bilaterally. Therefore, it seems that there is a bilateral symmetry in the number of roots in the first and second premolars of maxilla. Only in the maxillary second premolar, no cases of three-rooted teeth were seen on the left side; however, this was not statistically significant. Like previous researches [7,11,[34][35][36], no significant correlation was found between the number of roots and tooth positions in our study (p= 0.524).
Gender is one of the other factors, which can affect the morphology and number of roots [7,[23][24]. In our study, a significant relationship was shown between sex and root number in both premolars (p< 0.001), as the single-rooted teeth were dominant in females and tworooted premolars were more common in men.
In line with our study, Martin et al. [24] and Ok et al. [39] showed that females had significantly lower number of roots with less root canal system complexity compared to males. Celikten et al. [34], Alqedairi et al. [35], and Popovic et al [36] also found less root canal system complexity in females; however, these findings were not significant. In contrast to our study, Abella et al. [7] reported a similar number of roots between males and females. Considering the immensity of our country, the results of the present study cannot be applied to all Iranian population and it is suggested that further studies be conducted to investigate the number of roots and canal morphology in this group of teeth. Moreover, the relationship between tooth morphology with gender and tooth position needs further investigation.

Conclusion
This study can provide guidance to the root canals of ma-xillary premolar teeth for Iranian subpopulation leading to more optimal diagnosis and treatment planning for the endodontists. According to the findings, the complexity of root canal system and the number of roots were less observed in females compared to males.